1. Field of the Invention
The present invention relates to a method for detecting mis-insertion of an ATM cell and an ATM communication system, and more particularly to a method for detecting mis-insertion of an ATM cell, which can make early detection of mis-insertion of an ATM cell, of which error is hardly detected by an ATM network, and an ATM communication system.
2. Description of the Related Art
Generally, ATM (Asynchronous Transfer Mode) network is based on a connection type communication and assumes a single transmission path as a plurality of virtual transmission paths. Here, a transmission path which actually carries data is called a VC (virtual channel), and a path (route) which includes a plurality of transmission paths is called a VP (virtual path).
Routing (selection of a transmission route) of an ATM cell in the aforementioned ATM network is performed according to a conversion rule of a VCI (virtual channel path identifier) and a VPI (virtual path identifier) which are connection identification information temporarily given to a VC connection and a VP connection.
These VCI and VPI are defined by the header portion of the ATM cell, the connection is set by an ATM switch based on the VPI and VCI of the input cell, VPI/VCI are sequentially converted by a header conversion section in the ATM switch, and the ATM cell is transmitted by inserting the cell into the set connection.
FIGS. 6(a) and 6(b) are diagrams showing details of the format of the ATM cell. FIG. 6(a) shows a cell structure of a user-network interface (UNI), and FIG. 6(b) shows a cell structure of a network-node interface (NNI).
As shown in FIG. 6(a), the UNI cell header has four bits in the first half of a first octet which are general flow control (GFC) bits, a total of 8 bits of four bits in the second half of the first octet and four bits in the first half of a second octet which are virtual path identifier VPI bits, a total of 16 bits ranging from four bits in the second half of the second octet to four bits in the first half of a fourth octet which are virtual channel identifier VCI bits, first three bits in the fourth octet which are payload type (PT) bits, the remaining one bit in the fourth octet which is a cell loss priority (CLP) cell, and eight bits in a fifth octet which are header error control (HEC) bits.
Here, the HEC is comprised of Cyclic Redundancy Check (CRC) which is an error detection signal, an error which can be corrected by the HEC is one bit only, and the UNI cell and the NNI cell have a common structure excepting the presence or not of an GFC.
The transmission of data by the ATM is predicated on the use of a signal line, such as an optical fiber cable, having good line quality. In other words, it is assumed that the transmission data is substantially free from an error, and therefore, even the aforementioned HEC which is poor in the error detection and correction ability can keep satisfactory quality of transmission data.
However, when a radio transmission path is used for a part of the ATM network, lots of errors may be caused in the transmission data in this radio communication section depending on conditions that the weather is good or not, there is a disturbance or not, and the like.
Generally, a data error caused in the radio communication section is detected/corrected by an error correcting function of a radio receiver and sent as normal data to the ATM communication equipment, but because many data errors are easily caused in the radio communication section, there may be data of which error cannot be corrected by the error correcting function of the radio receiver.
When such error data of which error could not be corrected by the radio receiver is input to the ATM communication equipment, the error detection function of the ATM communication equipment operates.
And, if an ATM layer of the ATM communication equipment could detect an error (unset VPI/VCI or HEC error), the error data can be disposed, but if could not, the error data enters the ATM network and cannot be disposed until the error is detected by a higher-rank layer which is ranked higher than the ATM layer.
The more the detection of the mixture of the error data is delayed, the more an influence caused on the network becomes large. Even if the error is detected by the higher-rank layer, it is difficult to identify the cause of the error or the location where the error has occurred, so that it is not advisable to expect the detection of the error by the higher-rank layer.